1. Field of the Invention
The invention concerns a metering or dosing device for sand spreading apparatuses, especially for rail vehicles for spreading sand between wheel and rail.
2. Prior Art
In known sand-scattering apparatuses of this type the sand flows from the supply tank for the sand to the wheel through a housing with a throughflow borehole, wherein the amount of throughflow or the throughflow jet for the sand is controlled by a metering piston that is controlled electrically, pneumatically or in another manner and regulates the throughflow opening from 0 to a prescribed value. When the metering piston is closed, it sits with an end seal on the edge of the opening of the throughflow borehole and opens it when the piston is drawn back. The opening gap is pre-adjusted as a function of the coarseness of the sand used to avoid bridgings on the required minimum gap and thus also obligatory predetermines a random amount of sand in connection with the annular opening gap. However, this system was not completely satisfactory for certain sand demands, especially pneumatic sand demands because the sand qualities are adjustable only in relatively large tolerances so that defined small amounts of sand are attainable only unsatisfactorily with the known system in certain cases.